Various etching and deposition processes for semiconductor manufacture are performed in vacuum process chambers. For example, dry etching and chemical vapor deposition (CVD) processes utilize vacuum process chambers. Conventional dry etching processes include plasma etching and reactive ion etching (RIE). Conventional chemical vapor deposition processes include plasma enhanced chemical vapor deposition (PECVD) and low pressure chemical vapor deposition (LPCVD).
During these processes the process chamber can be evacuated from an initial pressure to an operating pressure. For example, the process chamber may initially be at atmospheric pressure for loading wafers, then evacuated to an operational pressure in the milli-torr range. The initial evacuation cycle for a process is sometimes referred to as a "pump down cycle". Typically, a pump down cycle is accomplished using a vacuum pump in flow communication with the process chamber.
Subsequently, the pressure in the process chamber can be increased from the operating pressure back to the initial pressure (e.g., back to atmospheric pressure). The subsequent pressurization cycle is sometimes referred to as a "vent up cycle". Typically, a vent up cycle is accomplished by injecting an inert gas into the process chamber to a desired pressure.
Recently, etching and deposition systems having more than one vacuum process chamber have been employed for semiconductor manufacture. These multi-chamber systems improve production rates and provide increased efficiency over single chamber systems. An example of a multi-chambered etching or deposition system is sold under the trademark "APPLIED MATERIALS 5000", by Applied Materials, Inc., of Santa Clara, Calif.
Such a multi chambered system can include a wafer handler, a load lock chamber and multiple process chambers. The wafer handler can include cassettes for holding the wafers and cassette ports for loading the wafers. During an etching or deposition process, the wafers can be moved from the load lock chamber and into or out of the process chambers as required. The process chambers can be pumped down and vented up to different pressures during various cycles of the process.
One limitation of multi chamber systems is that wafer defects can sometimes occur more frequently in a particular process chamber relative to the other process chambers. For example, some types of wafer defects can be detected using optical detectors such as those manufactured by KLA Instruments Corporation, Santa Clara, Calif. These types of defects are sometimes termed "KLA defects". The inventors have observed variations in KLA defects among wafers processed in different process chambers of multi chamber vacuum systems. In particular, some process chambers in multi chamber systems produce wafers with more defects.
One possible source of defect variation between the process chambers is that the rate of pressure change for the chambers during pump down and vent up cycles may not be the same. This difference in rate of pressure change can cause the pressures in the process chambers to be different for significant time increments. The pressure rate differences may be due to variations between conduction lines, pumps, valves and associated equipment for the different chambers. These variations can be caused by residue build up and other factors.
The same situation can occur among different single chamber systems adapted to perform the same process. Specifically, variations can occur between the different process chambers causing differences in the wafers. In this situation it would be advantageous to control the rate of pressure change during pump down and vent up in the process chambers in order to achieve process uniformity.
Prior art attempts to regulate pump down cycles in vacuum process chambers include "soft-start" valves, which open at a linear rate (i.e., at a certain percentage per second). Prior art attempts to regulate vent up cycles in vacuum process chambers include needle valves and mass flow controllers which control the flow rate into a particular chamber during vent up. However, these prior art systems do not compensate for system variables and are inherently linear in response. Accordingly, significant pressure differentials can still occur between different process chambers causing differences in the semiconductor wafers being processed.
The present invention provides a method and apparatus for achieving an optimal rate of pressure change in a vacuum process chamber during pump down and vent up cycles of a vacuum process. For multi chamber vacuum systems, the rate of pressure change between different process chambers can be matched such that one process variable can be eliminated and wafer uniformity can be improved. Similarly, for multiple single chamber systems adapted to perform the same process, one process variable can be eliminated and the uniformity of the wafers produced by the different vacuum process chambers can be improved.